1. Field of the Invention
The invention relates to light curing apparatus and processes for preparing dental restorations.
2. Description of Related Technology
Shrinkage of light- and self-cured dental composites used for direct, intraoral restorations has long been a concern of dental investigators because of the potential for micro-leakage to occur at a tooth-restoration interface. When composite restorations cure, they shrink as a result of the polymerization of the resin monomers in the composite. The volumetric shrinkage of composites may be in the range of approximately 3-4%. Micro-leakage resulting from such shrinkage can contribute to recurrent tooth decay, staining, and sensitivity.
Investigators have demonstrated that self-cured dental composites display less shrinkage stress than light-cured composites, because self-cured composites exhibit the ability to flow to allow the relaxation of stress during a relatively slow curing period, usually several minutes. (See Davidson et al, JDR, 63:1396-1399(1984); Feilzer et al., Dent. Mater., 9:2-5 (1993)). When a composite is light-cured it quickly polymerizes to a fairly high cross-link density, for example, within just a few seconds. Such quick polymerization does not provide sufficient time for the composite to relax and relieve the stress.
Light-induced polymerization shrinkage has long been a concern in the dental industry because of its potential to cause xe2x80x9cdebondingxe2x80x9d at the tooth-restoration interface, especially when adhesive strengths are not optimal. With an adhesive bond strength of sufficient magnitude, the polymerization shrinkage can be redirected to reduce the stress at the restoration-tooth interface. This redirected polymerization shrinkage, however, can create internal stresses in the restorative material (composite resin) or the remaining tooth structure detrimental to the long term success of the restored dentition.
The use of advanced dental adhesives may cause the fracture of the more brittle tooth enamel at the margin of the restoration in response to the polymerization shrinkage. As a result of the enamel fracture, micro-leakage can then progress to the detriment of the restored dentition. Enamel is an anisotropic brittle substance consisting mainly of rods or prisms, having a high elastic modulus and low tensile strength resulting in a very rigid structure. (See The Art and Science of Operative Dentistry, 3rd. Ed., C. M. Sturdevant, T. M. Robertson, H. O. Heymann, and J. R. Sturdevant editors, 1995, Mosby-Year Book, Inc., St. Louis Mo., pp. 12-18). Forces or stresses applied perpendicular to the direction of the enamel rods can much more easily fracture enamel as compared to parallel directed stress. The upper portion of Class I or Class II (MOD) restorations, above the dentin-enamel junction (DEJ), typically will have enamel rods parallel to the bonding line to be formed between adhesive and enamel. Because of the relatively weak nature of enamel, if stressed perpendicular to the rod direction, Class I or larger Class II restorations are susceptible to fractures within the enamel structure relatively close to the bond line forming cracks parallel to the bond line.
Of the several different classes of restorations, the Class I or Class II (e.g., MOD) type with their high amount of bonded surface area has in particular been the focus of much research due to its inherent high stress potential with good adhesives. Several studies on methods to counteract stresses in dental restorations have been conducted, including varied composite insertion methods into the dental cavity (e.g., bulk and incremental), composite shrink minimization, stress relaxation by flow to allow built-up stress to decrease, and strain measurements by numerous groups. See C. M. Kemp-Scholte and C. L. Davidson, J. Dent Res., vol. 67, p. 841, (1988); J. R. Bausch, et al., J. Prosthetic Dent., vol. 48, vol. 59 (1982); A. J. Feilzer, et al., J. Dent. Res., vol. 66, p. 1636; C. Davidson, J. Prosthetic Dent., vol. 55, p. 446 (1986); A. Feilzer, et al., J. Dent. Res., vol. 68, p. 48 (1989); C. M. Kemp-Scholte and C. L. Davidson, J. Prost. Dent, vol. 64, p. 658 (1990); C. M. Kemp-Scholte and C. L. Davidson, J. Dent. Res, vol. 69, p. 1240 (1990), C. L. Davidson, et al., J. Dent. Res., vol. 63, p. 1396 (1984); M. R. Bouschlicher, et al., Amer. J. Dent. vol. 10, pp. 88-96 (1997). It has been found that slow, self-cured composites reduce the rate of shrinkage stress formation which may cause enamel cracking.
In contrast, the visible-light curing process currently used with dental composites is a very fast, free-radical-initiated process where the bulk of the polymerization reaction typically is completed within just a few seconds. This quick process has always been considered a great advantage to the clinician but can lead to very high stress rates within the tooth structure. Such high rates of stress formation are known to cause premature failures in brittle materials (See Organic Coatings: Science and Technology, Vol. II, Z. W. Wicks, el al., John Wiley and Sons, New York, pp 105-31 (1994) and references cited therein; see also, An Introduction to the Mechanical Properties of Solid Polymers, I. M. Ward and D. W. Hadley, John Wiely and Sons, New York, Chapter 12 (1993) and references cited therein).
A study conducted by the inventors to illustrate stress formation problems with light-cured composites as compared to self-cured composites included the steps of placing an increment of a light-cure composite (LITEFIL(trademark), Bisco, Inc., Schaumburg, Ill.) into a Class I cavity of approximately 3xc3x973xc3x973 mm up to the dento-enamel junction. After a light cure of ten seconds at 600 milliwatts per square centimeter (hereinafter mW/cm2), a second increment of the same light-cure composite was added to the cavo-surface margin and the composite was cured using a high intensity light (e.g., ten seconds at 600 mW/cm2) on each of the buccal, lingual, and occlusal surfaces. An enamel crack was observed on one side of the restoration (see FIG. 4). The inventors performed an identical study using a dual-curing (self-light) dental composite (DUO-LINK, Bisco, Inc., Schaumburg, Ill.) for the second increment and allowing the self-cure process to proceed for five minutes before light curing. In this latter study, no enamel cracks were observed (See FIG. 5). No cracks were observed in dentin for either the light-cured or self-cured samples studied. (The reason why enamel cracked and dentin did not crack is most likely due to dentin""s higher flexibility (lower modulus). Thus, it was determined that the slower, self-curing composite could provide excellent structural integrity compared to light-cured composites. However, self-cured composites are not clinically ideal for use as occlusal surface restorative materials. Thus, for occlusal surface restorative applications, light-cured composites remain the most desirable option. However, latest trends with light curing are towards curing dental composites even faster, using higher and higher intensity curing systems, which can only create higher stressed restorations.
Commercial curing systems usually use fixed intensities, typically at 400 to 600 mW/cm2, which are not adjustable by the user. Additionally, commercial curing systems employ curing times that are usually pre-programmed at a minimum of ten seconds to a maximum of sixty seconds in ten second increments. Such curing systems require dental clinicians to use high curing intensities for durations which will result in high stress formation. Shorter than ten second duration curing times are not provided for by commercially-available curing systems.
Known devices used to cure visible-light-initiated dental composites are often called light-curing (LC) units or guns, since the hand-held portion of most units looks like a gun with a trigger for activating light. A typical visible light curing unit includes a base unit that typically sits on a counter in a dental operatory and houses the electronics that operate the light. The base unit may have a timer, some type of holder (also referred to as a cradle) for the gun, and an xe2x80x9coff-onxe2x80x9d switch. There is an electrical cord for plugging the base unit into a wall outlet, and an umbilical cord that attaches the base unit to the hand-held gun. Some LC units may include an on-board radiometer for periodically checking the output intensity of the light, with the readout typically being an LED. Such LC units have an aperture located on the base where a light tip (also referred to as a light guide) is placed. When the light tip is positioned accordingly in the aperture and the light is activated, a reading is registered, usually in mW/cm2 intensity units.
A typical visible light curing unit also includes a gun that houses the light bulb, cooling fan, finger trigger, and a port for the light guide. The cooling fan operates to dissipate heat generated by the light bulb, and operates for a variable amount of time after the light bulb has been turned off. Turning off the base unit prematurely before the cooling cycle is complete can possibly cause damage to the visible light curing unit. Light bulbs are manually accessed and replaced by disassembling part of the gun. The finger trigger when depressed activates the light bulb for a pre-programed time. Depressing the finger trigger a second time will interrupt the cooling cycle and deactivate the light bulb. Typical wavelength of visible light used in commercial typical light curing units is about 400-500 nanometers (nm).
The light tip or guide tip is a cylindrical extension residing in the port at the front end of the gun and exists to deliver the exiting light to the patient. Tips usually have a slight curve at its distal end for easy access and positioning into the oral cavity, and are commonly available in several different diameters. the most common being 2 millimeters (mm), 3 mm, 8 mm, 11 mm, and 13 mm. A removable amber-colored eye-protection-shield is usually slidably attached to the tip. This shield can be adjusted to the right or left side of the tip to provide the operators viewing protection. Tips may be detached for cleaning sterilizing.
An exemplary cordless visible light-curing unit is commercially-available under the name PROLITE from Dentsply/Caulk, Milford, Del. This unit includes a cordless gun, meaning the base does have an electrical cord, but the gun is not tethered to the base unit like most curing units. The base has three indicator lights: POWER, CHARGING, and READY. The amber-colored CHARGING light is activated (i.e. on) any time the unit has been used and is being re-charged. Once charged, the amber light is deactivated (i.e. turns off) and the green light is activated. There is no xe2x80x9coffxe2x80x9d switch on the base unit. The gun sits in a cradle that doubles as a charger and base unit. An on-board radiometer gives: (1) a green light if the power exceeds 300 mW/mm2, (2) an amber light if the power is 150-300 mW/mm2; (3) and a red light if the power is below 150 mW/mm2. A timer is located on the back of the gun handle and is displayed in a small LED window. Curing times can be set in ten second increments from 10-60 seconds. When the light is activated, the timer counts down to zero from the present amount of time. By setting the timer to zero, the gun can be used in a continuous curing mode. In the continuous curing mode, the timer counts up continuously to 120 seconds before automatically deactivating the light. As noted above, the curing unit is cordless, and, therefore, a battery is installed in the gun handle. It is recommended to completely discharge the battery once per month and then recharge the battery. A completely charged gun will cure for 14 full minutes prior to running out of power. Four consecutive beeps alert the user when the battery is about to run out of power. A 35 Watt light bulb is used having a life of 30 hours.
Another commercially-available cordless visible light curing unit is sold under the name VIVALUX II by Ivoclar-Vivadent, Amherst, N.Y. This cordless light-curing unit has two charging docks or receptacles. A typical three-light system warns the user of the charging status, wherein a green indicates that the unit is charged, a yellow light indicates that the unit is currently charging, and a red light indicates the unit requires service. The base unit has no xe2x80x9con-offxe2x80x9d switch. The gun for this unit has no on-board radiometer. The gun has a digital display on the top rear of the gun handle, which displays an icon that looks like a battery when the gun is fully charged. When the gun is discharged, the battery icon appears almost empty and blinks. This unit has no timer, there is one beep after 20 seconds, two beeps after 40 seconds, and three beeps after 60 seconds. Maximum curing time without recharging is 8.5 to 9 minutes. This unit has a 35 Watt light bulb having a life of 1000 hours.
An exemplary commercially-available light-curing unit that has a cord is OPTILUX 500, which is made and sold by Demetron/Kerr, Danbury, Conn. The base unit for this corded, light curing unit has a built-in radiometer for checking power output of the light. The radiometer is read via an LED numerical readout. A soft-touch control pad is located on a face of the base unit. The pad can be used to program various time cycle settings for light exposure. The settings include 10, 20, 30, 40, 50, 60 seconds cycles or continuous curing. A first switch on the back of the base enables the operator to set a beeping signal to sound every 10 or 20 seconds as well as beeping at the end of a preset cycle. A second switch enables the operator to control the volume of the beeping signal, and a third switch is used for resetting the total elapsed hours of the current bulb when the light bulb is replaced. A finger switch on the gun activates and deactivates the light bulb. A preset timing is used to automatically deactivate the light bulb source at the completion of that preset time period. An 80 Watt halogen bulb is used in the gun. The base unit can be wall-mounted if so desired.
Another commercially-available, corded, light-curing unit is available under the name OPTILUX 401/403, from Demetron/Kerr, Danbury, Conn. The base unit of this light-curing unit has a circular know-shaped timer positioned on a top surface which allows time cycle presetting of 10, 20, 30, 40, 50 and 60 second exposures. A beeping signal signifies the end of a preset time period. An 80 Watt bulb is used in the gun, the bulb having a life of about 80 hours.
Another commercially-available, corded light-curing unit is sold under the name SPECTRUM by Dentsply/Caulk. Milford, Del. The base unit of this light-curing unit consists of a built-in radiometer and a removable cradle that can be mounted or placed in a convenient location. Optionally, the base unit can be wall mounted. Instead of an LED readout for the radiometer, the base unit uses three lights to indicate the status of curing power, wherein: (a) a green light signifies acceptable power, (b) a yellow light signifies caution, (c) and a red light indicates that replacement or service is needed. A short sounding beep signifies the beginning of the curing cycle and a long beep indicates the end of the cycle. Available time cycle settings include: 10, 20 or 60 seconds. The gun uses a 49 Watt light bulb having a life of 20 hours.
Another commercially-available, corded light-curing unit is sold under the name XL3000 by 3M, St. Paul, Minn. The base unit of this light-curing unit can be wall-mounted or placed on a counter top. A swing out door houses a spare bulb for convenience. The gun was a 75 Watt lightbulb having life of 40 hours. An on-board radiometer causes illumination of a green light if the power output is acceptable; if power is unacceptable, no illumination occurs. A single beep indicates both the beginning and end of a cure cycle. A double-beep occurs if the cure cycle has been manually interrupted. A timer is located on the back handle of the gun, having time cycle settings of 10, 20, 30 40 seconds or XT (for extended curing for a maximum of 200 seconds). A beep sounds every ten second during an XT time cycle setting.
Another commercially-available, corded light-curing unit is sold under the name ELIPAR HIGHLIGHT by ESPE, Norristown, Pa. This unit features a standard setting with full power (700 mW/cm2) and a second time cycle setting with a xe2x80x9c2-Stepxe2x80x9d mode. In the 2-Step mode, the light intensity is at low power (150 mW/cm2) during the first 10 seconds and then immediately switches to full power (700 mW/cm2) for the remaining 30 or 50 seconds of curing depending on user requirements. An advertisement for the unit appearing in DPR Europe, November 1996, states that the initial low power is used xe2x80x9cto minimize the risk of fracture and optimize marginal adhesion, thus maximizing the life of the final restoration. The initial low light intensity of 150 mW/cm2 reportedly extends the plastic (low viscosity) phase of the material, allowing material stresses to balance out.xe2x80x9d ESPE product literature states that xe2x80x9c[i]n the 2-Step Mode, the lighting intensity is reduced during the first 10 seconds to reduce stress during initial and final curing of the material.xe2x80x9d There are four curing time cycle settings available in the standard mode: 20, 40 and 60 seconds and xe2x80x9c2-Stepxe2x80x9d mode. The 2-step mode is allowed to function only if 40 second or 60 second time cycles are chosen. This unit features several custom-programmable beeping functions. For example, the unit can sound a beep when the light is activated or turned off, beeping can occur at 10 seconds, then two beeps at 20 seconds and 60 seconds. A radiometer built-in to the base unit activates a green light to indicate adequate power, and a white light to indicate inadequate power. Since this unit has a 2-step mode for different curing powers, a microprocessor monitors the voltage supply. This guarantees constant light power throughout the entire life of the bulb. The bulb is 75 Watts, no life-expectancy is listed. It is noted that the ELIPAR HIGHLIGHT product literature does not state that the rate of stress formation is reduced; it states only that stress is reduced. There is no clear evidence the ELIPAR HIGHLIGHT light-curing unit reduces stress. Also, according to such a system, an operator cannot adjust or change the time cycle setting to cure at low intensity when using the 2-step mode. Also this light-curing unit provides no opportunity to pause between low and high intensity curing.
Another commercially-available corded, light-curing unit is sold under the name COLOTOLUX 4 by Coltene/Whaledent, Mahwah, N.J. The base unit has a ring-like cradle for the gun. A green LED on-off switch is located on the base. A timer, using a large dial, is located on the base and allows selection of 10, 20, 30, 40, 50 and 60 second curing intervals. The built-in radiometer has an LED readout. When the gun is activated, a beep sounds at the beginning of the curing cycle. The gun uses a 75 Watt light bulb having a life of 28 hours.
Listed below by manufacturer are additional commercially-available, corded intra-oral light curing systems
Listed below by manufacturer are additional commercially-available, cordless intra-oral curing systems.
It is an object of the invention to overcome one or more of the problems described above.
It also is an object of the invention to put the concept or stress-relaxation known from self-cure composites into practice in the art of light-cure dental restoration. This is only feasible if a far less than complete (e.g., partial) curing has occurred prior to allowing the relaxation to occur, and if the relaxing times required for the process are short enough to be reasonable for a dental clinician. Described herein is a composite curing process acceptable to the standard dental clinician and a visible-light curing unit design which will allow for the generation of lower rates of initial stress formation in Class I or larger Class II dental restorations, resulting in far reduced enamel fractures and more sound restorations.
A process according to the invention includes the steps of applying a composite restorative material onto a prepared tooth followed by the application of visible light (having a wavelength of approximately 400 to 500 nanometers) to the composite of intensity and time sufficient to penetrate the composite to initiate polymerization. Light application is then suspended for a period of time sufficient to allow for the relaxation of internal stresses created by the initial polymerization of the composite. Light, usually of a higher intensity, is subsequently applied to the composite to complete polymerization.
Also according to the invention, a curing light is utilized which includes multiple power and timing settings.
Other objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description taken in conjunction with the drawings and the appended claims.